Maybe it's useful to mention that the root of this verb being ق ي س , it is listed in the dictionary after words beginning with ق ي ر , but it is written as usual in the 3rd person past/perfect قاس . So in MSA it is conjugated this way : قِستُ I measured , you measured قِست , he measured قاس , she measured قاست , we measured قسنا, you ( masc.) measured قسْتُم , you (fem.) measured قستُنً , they ( masc.) measured قاسوا , they (fem.) measured قاسْن . It's a concave verb, i.e. the middle letter of the root is a semi-consonant ي , hence these irregularities. The verb to sell باع / يبيع is conjugated in the same way.